I have one of these on my PC. It's very skinny for the capacity. Cyberpower has quite a few relatively skinny UPSs, and Frys has several of the recertified ones at really good prices (see this thread). Looks like the one I have is sold out, but you could try some of the others. I also have one of their 1200va units (without the LCD) for the HR21 & TV in the kitchen, fits in the cabinet nicely, I'd say it's close to 4.5", but maybe not as thin as the LCD model.
Really you could get any rack-mount UPS that is 2U high. They make "tower" adapters to set them on end, just need to remove the metal "ears" used for rack mounting. APC has a tower adapter for their 2U rackmount units. Their rackmounts exhaust out the back so space is not as big an issue as the ones that have fans on the side.
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